1. Field of the Invention
Example embodiments of the present invention generally relate to an image forming apparatus and a method of controlling a development electric field strength generated therein, and more particularly, to an image forming apparatus such as a copier, printer, or facsimile machine, in which an electrostatic latent image is developed into a toner image by attracting toner to adhere to the electrostatic latent image by action of a development electric field formed between the electrostatic latent image formed on an image carrier and a surface of a developer carrier, and a method of controlling a development electric field strength generated between a development gap formed by a development sleeve and a photoconductor incorporated in the image forming apparatus.
2. Discussion of the Related Art
Known electrophotographic image forming apparatuses generally control electric potential setting values at given intervals to adjust various electric potentials such as charge target potentials and developing biases, so as to control image quality factors such as image density. In this adjustment of potential setting values, multiple reference toner images that serve as predetermined adjustment toner patterns are generally formed on the surface of a latent image carrier such as a photoconductor, and then the toner adhesion amount of each of the reference toner images is detected. Then, based on a linear approximation equation of a development electric potential in forming the reference toner images and the toner adhesion amount of each reference toner image, a development electric potential is calculated to obtain a desired toner adhesion amount, and various potentials are calculated to obtain that desired development electric potential. The term “development electric potential” means a difference between a potential of an area on the surface of the latent image carrier where the electrostatic latent image is formed and a surface potential of a developer carrier to which a developing bias is applied.
With such image forming apparatuses, a cylindrical developer carrier and a cylindrical latent image carrier are generally disposed facing each other across a given gap, hereinafter referred to as a “development gap”, and are driven to rotate separately. By the action of a development electric field generated according to the development electric potential, toner attracted to the surface of the developer carrier is transferred onto the surface of an electrostatic latent image formed on the latent image carrier so that the electrostatic latent image is developed into a visible toner image. It is to be noted that, with the above-described configuration, the developer carrier and the latent image carrier may cause respective runouts on the outer circumferences thereof with respect to respective rotation drive shafts. The term “runout” refers to variation or out-of-round on the circumference of the developer carrier and that of the latent image carrier.
Here, if the development gap is represented by “d [m]” and the development electric potential that can be obtained by the above-described potential setting value control is represented by “Vpot [V]”, then a development electric field strength “E [V/m]” can be obtained by Equation 1 described below:E=Vpot/d  Equation 1.
As the developer carrier and the latent image carrier rotate, the runouts on the outer circumference thereof cause the development gap to fluctuate, which in turn can cause the development electric field strength E to fluctuate. When the development electric field strength E fluctuates, an amount of toner that is transferred onto the electrostatic latent image formed on the surface of the latent image carrier also fluctuates. Therefore, the fluctuation in the development gap due to the runouts on the outer circumferences of the developer carrier and the latent image carrier can cause image density nonuniformity (hereinafter simply “density nonuniformity”) of an output image, resulting in a defective image.
To eliminate such density nonuniformity, improvement in runout accuracies of the developer carrier and the image carrier is required. With increasing market demand in recent years for an increase in speed of electrophotographic image forming apparatuses, developer carriers and image carriers have tended to increase in diameter size, which makes it difficult to improve the runout accuracies thereof. Further, even if the runout accuracies are successfully improved, it is likely that such improvement is accompanied by an increase in the cost of the apparatus.
Various techniques have been proposed to correct density nonuniformity caused by fluctuation in the size of the development gap. One technique, for example, discloses that density nonuniformity caused by runout in a developer carrier can be eliminated by adjusting the speed of rotation of the developer carrier. However, this approach does not take into account runout in an image carrier, and therefore is not wholly sufficient to suppress the density nonuniformity caused by fluctuation of a development gap.
Further, the above-described technique needs to obtain the current density nonuniformity, and therefore, in addition to multiple reference toner images formed when adjusting conventional potential setting values, other reference toner image(s) may need to be formed on the latent image carrier. Accordingly, problems such as an increase in downtime of the image forming apparatus along with an extended time required for implementing the potential setting value control and an increase in toner consumption amount remain.